Ventilatory assistance using an external effort sensor

ABSTRACT

The ventilator of the invention uses a respiratory effort sensor that does not rely on respiratory airflow or pressure for synchronization, such as a device that measures movement of the suprasternal notch in response to respiratory efforts. This makes the ventilator relatively immune to leaks. Furthermore, the mask pressure may be modulated so as to servo-control the respiratory effort signal to be zero. Because effort is servo-controlled to be near zero, it is not necessary for the effort signal to be either linear or calibrated, but merely monotonic on effort. Similarly, it is possible to achieve near 100% assistance without having to know or estimate the resistance and compliance of the patient&#39;s respiratory system, as in those systems that provide proportional assist ventilation

FIELD OF THE INVENTION

[0001] This invention pertains to the delivery of mechanicalventilation, and more particularly to the delivery of noninvasiveventilatory support for patients with lung disease.

BACKGROUND

[0002] A goal of ventilatory support is to reduce the internal work ofbreathing in subjects with disease of the lungs or chest wall. Typicalmechanical ventilators deliver a varying pressure to the airway, forexample, via a face mask or nose mask. In a spontaneously breathingpatient, a fundamental problem that must be solved is thesynchronization of the delivered pressure with the patient's spontaneousefforts. For example, in a typical bilevel ventilator, the mask pressureis switched to a higher pressure, for example, 20 cmH₂O, at the momentof detection of patient inspiratory airflow, and a lower pressure, forexample, 5 cmH₂O, at the moment of cessation of patient inspiratoryairflow. There are at least four problems with the use of patientrespiratory airflow to trigger the device between the higher and lowerpressures:

[0003] (1) large and varying leaks cause a discrepancy between patientrespiratory airflow (the desired quantity) and mask flow (the measuredquantity);

[0004] (2) dynamic airway compression and intrinsic positive pressurecause a mis-match between effort (the truly fundamental quantity) andairflow (the measured quantity);

[0005] (3) cardiac emptying produces airflow that can be confused withrespiratory airflow; and

[0006] (4) in patients with high airway resistance causing longrespiratory system filling and emptying time constants, inspiratoryairflow can continue after cessation of inspiratory effort, andexpiratory airflow can continue despite recommencement of inspiratoryeffort.

[0007] There are several known ways to find a more direct measure ofpatient respiratory effort than is afforded by respiratory airflow. Onemethod is to invasively measure intrathoracic pressure, for example, byplacing a pressure sensing catheter in the oesophagus. Increasingrespiratory effort produces increasing subatmospheric pressure at thesensor. However, this method is too invasive for general use. Anothermethod is to place respiratory movement sensors around the chest wall,typically one around the thorax and one around the abdomen. Suitablesensors use either inductance pneumography, although reasonable signalscan be obtained using strain gauges, magnetometers, graphite-in-rubberbands, etc. Unfortunately, none of these methods truly measuresrespiratory effort as opposed to the resulting chest wall movement, andif the airway is partially or completely occluded, then the signal cangreatly diminish in amplitude even though the respiratory effort hasactually increased. Also, changes in mechanics with body position orsleep state make these prior art sensors unreliable.

[0008] A third method is to use a diaphragm electromyogram as a measureof spontaneous effort, but this is extremely technically demanding,unsuitable for long term use, and difficult to process due to cardiacartifact. A fourth method, taught by McWilliams in U.S. Pat. No.5,513,631, is to monitor movement of the nostrils, which flare somemoments prior to commencement of spontaneous inspiration under somecircumstances. Unfortunately, this may not occur in all sleep states, orunder conditions of relatively normal respiratory drive, making it lessuseful as the condition of concern becomes partially treated.

[0009] Another fundamental problem is to determine how much support togive, for example, whether to vary the pressure by 15 cmH₂O as in theabove example, or a smaller or larger value. Various known methodsattempt to tailor the degree of support to suit the patient. One methodis to deliver a fixed degree of support using a bilevel ventilator, andto attempt to tailor that degree of support to be best on average forthe patient, by aiming to strike a balance between optimizing arterialoxygenation or carbon dioxide level versus providing comfort to thepatient. An advantage is that patients are free to take larger orsmaller breaths, and at varying rates, which helps with comfort, but adisadvantage is the inability to provide suitable support under varyingconditions. Another essentially opposite method is to provide a fixedvolume of air per breath (volume cycled ventilator) or a fixed volumeper minute (servo ventilator), which is more effective but lesscomfortable. A pressure support servo ventilator, for example, as taughtin PCT Application 97/00631 of Berthon-Jones entitled “AssistedVentilation to Match Patient Respiratory Need”, combines the comfortadvantages of a pressure support ventilator with the blood gasoptimizing advantages of a servo ventilator (as well as other advantagesrelated to patient-machine synchronization). Still another method,proportional assist ventilation, is described in Younes U.S. Pat. No.5,107,830. Proportional assist ventilation measures patient respiratoryairflow, and provides support for the resistive component of work ofbreathing proportional to the respiratory airflow, and support for theelastic component proportional to the integral of respiratory airflow.All the above methods still have varying degrees of problems associatedwith the difficulties of measurement of respiratory airflow underconditions of high leak.

[0010] A ventilator, in general, comprises three parts:

[0011] (1) a respiratory effort sensor, or a sensor of a surrogate ofrespiratory effort,

[0012] (2) a source of breathable gas at a controllable pressuredelivered via a mask or similar interface to a patient's airway, and

[0013] (3) a control that modulates the mask pressure so as to reducethe respiratory effort signal.

[0014] Generally, the respiratory effort signal is inferred from asurrogate such as mask pressure or flow. Both are unreliable indicatorsof effort in the presence of leaks, particularly changing leaks, whichare particularly ubiquitous during noninvasive ventilation. They arealso unreliable indicators in the presence of intrinsic PEEP (PositiveEnd Expiratory Pressure), which is common in severe lung disease,because effort can precede flow by an appreciable period. They are alsounreliable in the presence of a long respiratory time constant, as isuniversal in obstructive lung disease, because machine-induced pressurechanges lead to long-lasting flow changes even when the patient hasceased inspiratory effort. More direct measures of respiratory effortknown to be useable for the control of ventilators are either invasiveor unreliable or both.

BRIEF DESCRIPTION OF THE INVENTION

[0015] The present invention preferably uses a respiratory effort sensorthat measures movements of the suprasternal notch in response torespiratory efforts. This effort sensor is disclosed in co-pendingapplication Ser. No. ______, entitled “Measurement of Respiratory EffortUsing a Suprasternal Sensor” and filed on even day herewith.

[0016] The first advantage over the use of respiratory airflow or maskpressure as an indicator of respiratory effort is immunity from leak,resulting in better patient-machine synchronization. The secondadvantage is that the effort signal increases very promptly after onsetof muscular inspiratory effort, even in the presence of intrinsic PEEP,again resulting in better patient-machine synchronization. The advantageover the use of measures such as oesophageal pressure or diaphragmelectromyogram using oesophageal electrodes is non-invasiveness. Theadvantage over surface diaphragm or alae nasi electrodes is robustnessand ease of use. The advantage over the use of other indirect measures,such as respiratory movement sensors or impedance sensors on the chest,is robustness and ease of use. The chest has at least two degrees offreedom, making such measures unreliable.

[0017] The invention also entails use of a novel control mechanism, inwhich the mask pressure is modulated so as to servo-control therespiratory effort signal to be zero. The respiratory effort signal isany respiratory effort signal that is not derived from respiratoryairflow or mask pressure as in the prior art, i.e., any respiratoryeffort signal that is not a function of measurement on the air breathedby the patient, the preferred sensor being the suprasternal notch sensormentioned above.

[0018] The method of the invention provides all the advantages ofproportional assist ventilation, specifically, very precise matching ofventilatory support to respiratory effort under ideal conditions of zeroleak and no intrinsic PEEP. In addition, the method is immune to leak,resulting in better patient-machine synchronization. Furthermore,because effort is servo-controlled to be near zero, it is not necessaryfor the effort signal to be either linear or calibrated, but merelymonotonic on effort (a general requirement of any servo control).Similarly, it is possible to achieve near 100% assistance without havingto know or estimate the resistance and compliance of the patient'srespiratory system, as in those systems that provide proportional assistventilation.

BRIEF DESCRIPTION OF THE FIGURES

[0019]FIG. 1 depicts an optical sensor 107 mounted on the sternum 106,so that the output signal is influenced by the movement of the skin ofthe suprasternal notch 101;

[0020]FIG. 2 shows a graph of light current from a commerciallyavailable optical sensor as a function of distance between the front ofthe sensor and the skin. The graph shows how if the sensor is placed atapproximately 8 mm from the skin, the signal decreases as the skin issucked away from the sensor with inspiratory effort, and the signalincreases as the skin bulges towards the sensor with expiratory effort;

[0021]FIG. 3 is a block diagram of processing electronics, including atrough detector to track changes in the light signal even in thepresence of body movement, and a subtractor to remove such changes fromthe signal;

[0022]FIG. 4 shows the response of the processing electronics of FIG. 3to a shift in the signal at minimum effort due, for example, to bodymovement; and

[0023]FIGS. 5 and 6 comprise the illustrative embodiments of theinvention, in which

[0024] the block diagram circuit of FIG. 5 uses the preferredrespiratory effort sensor of FIGS. 1-4 (shown generally by the numeral300) to produce a signal 301 that is an increasing function of effortand that is compared with a threshold level 302 (indicated by the dottedline on signal 301) by trigger circuit 303 to produce a pressure requestoutput signal 304, and

[0025] the pressure delivery system of FIG. 6 includes a pressurerequest input 400 (from FIG. 4) to servo 401 whose output 402 controlspressure source 403 that delivers breathable gas via hose 404 to mask405 and exhaust vent 406, with pressure sensor 408 measuring maskpressure via hose 407, and the output 409 from the pressure sensorproviding feedback to the servo 401.

DETAILED DESCRIPTION OF THE INVENTION

[0026] 1. Suprasternal Notch Optical Effort Sensor

[0027] The preferred embodiment of the present invention uses an opticalsensor, such as an infrared proximity sensor, to measure the depth ofthe suprasternal notch, as shown in FIG. 1. A light source 100 shineslight on the skin of the suprasternal notch 101, and the reflected lightis received by photocell 102. (The term “photocell” is used to refer toany device whose output is light sensitive, e.g., a photodiode,phototransistor, etc.) The combined sensor assembly 107 may be mountedon any surface which is relatively immobile with respect to the skin ofthe suprasternal notch, such as the sternum 106. A suitable method forattachment is to mount the sensor 107 on cantilever 103, which is thenglued to the sternum using double-sided adhesive tape 104.

[0028] Preferably, the double-sided adhesive tape is not glued directlyto the skin, but is glued to a layer of soft, spongy, low irritant, lowallergenic self-adhesive material 105. A suitable material is DuoDERM®,available from ConvatTec, of Princeton, N.J. The sensor is mounted suchthat the optical paths of light source and photocell are approximatelynormal to and centered over the skin at the deepest point of thesuprasternal notch. The advantage of the layer of DuoDERM is that it canremain in place on the skin for long periods, and the sensor can beremoved and reapplied multiple times without trauma to the skin.Cantilever 103 can itself be made from a semi-flexible material, such asfoam or silicone rubber with embedded aluminium reinforcing, so that itcan be bent to conform to suit the subject and adjust the distance fromthe skin of the suprasternal notch. Alternatively, or in addition,sensor 107 can be mounted on the cantilever with adjusting screws so asto adjust the distance of the sensor from the skin of the suprasternalnotch. To gain a lower profile, it is convenient to have the opticalaxis of the sensor parallel with the sternum and use a small mirror todirect the light path at the skin of the suprasternal notch. Another lowprofile arrangement is to surface mount the sensor electronic componentsdirectly onto the cantilever.

[0029] Small to moderate inspiratory and expiratory efforts causequasi-linear movement of the skin of the suprasternal notch, withinspiratory efforts causing the skin to be sucked inwards, away from thesensor, and active expiratory efforts to cause the skin to bulgeoutwards, towards the sensor. Progressively larger efforts causeprogressively smaller increments in skin movement, and efforts of morethan about ±10 to 20 cmH₂O pleural pressure produce little furtherchange in the signal. This is convenient, because small efforts producemeasurable deformations in the skin, and it is desired to detect smallefforts.

[0030] In a typical arrangement, the light source 100 of the sensor isan infrared light emitting diode, and the photocell 102 of the sensor isa photoresistor, photodiode, or phototransistor. For example, using acommercially available EE-SF5 photomicrosensor available from OmronCorporation, of Kyoto, Japan, the electrical output (light current)increases quasi-linearly for distances from zero to about 4 millimetres,and then decreases quasi-exponentially for distances greater than 5millimetres, as shown in FIG. 2. (At short distances, a reduced amountof light is detected because of light angle considerations.) Therefore,in the preferred embodiment, the sensor assembly is placed so that thefront face of the combined sensor 107 is approximately 8 millimetresfrom the skin. Inspiratory efforts will cause the distance to increase,resulting in a quasi-exponentially decreasing electrical signal, andexpiratory efforts will cause an increasing signal.

[0031] In an alternative embodiment, the sensor could be positioned andsized such that it is the ascending portion of the curve of FIG. 2 thatis operative, with the light current output increasing with increasingdistance.

[0032] It is also possible not to glue the cantilever to the skin, butto hold it in place using a bandage, harness, or similar mechanism.Alternatively, the cantilever may be attached to a tight stretch garmentsuch as a Lycra® T-shirt. Combining both alternatives, the cantilevermay be mounted on a large disc of soft, thin, high-friction materialsuch as silicone, typically 10 centimeters in diameter, which may beheld by friction in contact with the skin by a harness, bandage, stretchLycra T-shirt, etc. A very low durometer silicone will tend to have ahigher coefficient of friction. The large, soft, thin, disc of highfriction material may be perforated with multiple holes in order toallow the skin to breathe.

[0033] Changes in body posture, for example, turning the head, extendingthe neck, or rolling from back to side, can change the depth of thesuprasternal notch independently of respiratory effort. Therefore, it isdesirable to be able to automatically maintain the signal correspondingto zero effort to be zero, independently of (non-breathing related) bodychanges.

[0034] Normally, inspiratory effort is active and expiration is passive.In the preferred embodiment discussed so far using the EE-SF5 sensor,inspiratory effort causes a decreasing light current, as shown in FIG.2. Therefore, for convenience, the output from the photosensor 102 isinverted, so that inspiration produces a positive signal. This signal isthen amplified and zero-adjusted so that zero effort produces an outputsignal of zero. Changes in posture will tend to change the distancebetween sensor and skin, which will change the output voltage for zeroeffort. It is desirable to automatically adjust for such changes inposture, so that zero effort once again produces zero output signal.

[0035] If the optical sensor has been set up so that a positive signalcorresponds to inspiratory effort, and if the patient is not makingactive expiratory efforts, the minimum signal during a breath willcorrespond to zero effort. A trough detector, comprising a capacitorcharged by the sensor output via a resistor, and discharged by thesensor output via a diode, with the resistor-capacitor time constantlong compared with a breath but short compared with the interval betweenpostural changes, will track this minimum effort. A suitable timeconstant is ten seconds. Preferably, the diode is in the feedback loopof an operational amplifier to provide correct operation close to zerosignal. A subtractor operational amplifier then subtracts the output ofthe trough detector from the output from the sensor to yield the effortsignal.

[0036] A suitable circuit block diagram for the entire assembly is shownin FIG. 3. Point (A) is the output from a phototransistor or otherlight-responsive detector, point (B) is the output after inversion byinverter 201, point (C) is the output from the trough detector 202, andpoint (D) is the zero-corrected effort signal output. FIG. 4 shows theaction of the entire assembly. The top tracing is the true respiratoryeffort, as might be measured using an esophageal pressure transducer,recorded for a period of 4 minutes, or 60 breaths. The peak inspiratoryeffort varies in amplitude with a period of 30 seconds. The secondtracing shows the signal from the phototransistor, at point (A). Thissignal is upside down, because increasing effort causes the skin torecede from the sensor, causing a reduction in light current from thephototransistor. Thus, zero effort is represented by the flat upperenvelope of the waveform at the leading and trailing ends of thetracing.

[0037] During the second of the four minutes, the DC offset changes, tosimulate the effect of a change in posture leading to the sensor beingheld closer to the skin (more light output) at zero effort. The thirdtracing shows the signal at point (B), after inversion. Here, zeroeffort is represented by the flat lower envelope of the waveform at theleading and trailing ends of the tracing. The heavy line on the fourthtracing shows the signal at point (C), which is the output of the troughdetector. For convenience, the signal at point (B) is reproduced as athin line along with the output of the trough detector. The troughdetector tracks the DC shift in the signal during the second minute ofthe tracing. The reason for this is as follows.

[0038] Consider that capacitor 206 has charged through resistor 205 tothe potential at point (B). If the potential at point (B) rises abovethat of the capacitor, the potential at the output of operationalamplifier 207 will be greater than that of the capacitor, and diode 207will be reverse biased. The capacitor potential rises slowly throughresistor 205 to the potential at point (B), but it takes several breathsfor this to happen. But if the input at point (B) drops below thecapacitor level, operational amplifier 207 conducts current through thediode. The capacitor voltage thus decreases rapidly to the lowest levelof the input.

[0039] The output at point (D) is shown in the bottom tracing—it is thedifference (formed in subtractor 203) of the signal at point (B) and theheavy line shown in the tracing for the signal at point (C). The netresult is that the final output signal at point (D) is zero for zeroeffort (along the horizontal axis of the tracing), even if the lightoutput changes due to a change in posture, and the signal increases withincreasing effort.

[0040] The above functionality can also be performed by a microprocessorwhich executes a program that samples the sensor output signal, tracksthe minimum signal over a moving time window long compared with a breathbut short compared with the interval between body movements (such as 10seconds), and subtracts the minimum signal from the sensor output signalto yield the effort signal.

[0041] 2. Triggering Using the Suprasternal Notch Sensor

[0042] The signal from the optical sensor may be used to trigger aconventional ventilator instead of the ventilator's usual triggeringmeans. In one embodiment, the effort sensor is combined with a prior artspontaneous mode bilevel ventilation control—the mask pressure is set toa high pressure (such as 20 cmH₂O) if the effort signal exceeds athreshold, and set to a low pressure (such as 4 cmH₂O) otherwise. Ablock diagram of such an arrangement is shown in FIG. 5. Effort sensor300 (the device of FIGS. 1-4) supplies zero and body position correctedeffort signal 301 to trigger circuit 303, which produces pressurerequest output signal 304. As shown by the two functions depicted in thedrawing, if the input exceeds the threshold on conductor 302, thepressure request signal is set to a high value, and to a low valueotherwise. These two values control the two pressures of a conventionalventilator.

[0043]FIG. 6 shows a block diagram of a servo-controlled pressuregenerator and air delivery system controlled by the same pressurerequest output signal 304. The pressure request signal is fed to thepressure request input 400 of servo 401, whose output 402 is used tocontrol a controllable pressure source 403 (such as a blower withvariable speed motor or control valve, or compressed gas and controlvalve). Air (which may be enriched with oxygen) from the controllablepressure source is fed via hose 404 to mask 405 and ultimately ventedthrough exhaust 406. A pressure sensor (transducer) 408 measures maskpressure via hose 407, and the electrical output 409 from the pressuresensor is fed back to the servo 401.

[0044] When the patient commences inspiratory effort, the pressure inthe pleural cavity falls, causing the effort signal to exceed thethreshold, and the higher pressure is applied. In patients with severeobstructive lung disease, requiring relatively high pressures, theintrathoracic pressure will remain negative during the patient'sinspiration, because the mask pressure is not immediately transmitted tothe alveoli. When the patient ceases making inspiratory effort at theend of inspiration, the intrathoracic pressure will suddenly rise,making the effort signal go back to zero, or even negative in the caseof actively exhaling patients. At this point, the effort signal dropsback below the threshold, and the device selects the lower pressure,permitting expiration to occur.

[0045] The same type of effort sensor signal can be used to triggerstill another prior art controlled ventilator, one which exhibits whatis known as spontaneous plus timed backup bilevel ventilation. With sucha ventilator, the mask pressure is switched to the higher pressure ifthe effort signal goes above a threshold indicating start of activeinspiration, and then switched back to the lower pressure when theeffort signal falls below the threshold indicating end of activeinspiration, as described above, but in the event that the start ofactive inspiration is not detected within a specified time from thestart of the previous active inspiration (or alternatively, within ashorter specified time from the end of the active inspiration), amachine generated “timed” breath is delivered by switching to the higherpressure for a specified duration.

[0046] In general, the output of trigger 302 can be used to replace thetrigger of any known class of ventilator, switching the ventilator fromthe expiratory to the inspiratory sub-cycle when the effort signal goesabove a threshold, or from the expiratory to the inspiratory sub-cyclewhen the effort signal goes below a threshold, or both. Such ventilatorsinclude but are not limited to volume cycled ventilators, pressuresupport ventilators, volume servo-ventilators, and proportional assistventilators.

[0047] 3. Degree of Support Proportional to the Effort Signal, asMeasured Directly Using the Effort Sensor: Effort Reducing VentilatorySupport

[0048] Thus far, what has been described is the triggering of aconventional control means between an inspiratory and an expiratorystate. A further aspect of my invention relates to adjusting the degreeof support to be proportional to the effort signal.

[0049] The output E(t) on conductor 301 from the effort sensor 300 inFIG. 5 may be delivered to an amplifier which generates a pressurerequest signal P(t), such that

P(t)=P ₀ KE(t).   (Equation 1)

[0050] This is the desired pressure, and if is equal to a bias level P₀plus a pressure that is proportional to the patient's effort. Thepressure request signal P(t) is then applied to the pressure requestinput 400 in FIG. 6. (An actual circuit for implementing this embodimentof the invention is identical to that of FIG. 5, with the differencethat trigger circuit 303 is replaced by a circuit for generating thefunction of Equation 1.)

[0051] It is instructive to compare this embodiment of the invention(effort reducing ventilatory support) with conventional proportionalassist ventilation. Proportional assist ventilation provides aninstantaneous pressure which is a function of airflow f(t), as follows:

P(t)=P ₀ +Rf(t)+E∫f(t)dt, f(t)>0  (Equation 2)

P(t)=P ₀ +Rf(t), otherwise.  (Equation 3)

[0052] In these equations, R is the resistance of the subject's airway,and the product Rf(t) represents a desired pressure component whichcompensates for the way the airway impedes air flow. The term E∫f(t)dtrepresents a desired pressure component which compensates for theelastic recoil of the patient's lungs.

[0053] A first difference between the effort reducing ventilatorysupport of the invention and the prior art proportional assistventilation is that there is no need to measure respiratory airflowf(t), with its attendant problem of leaks.

[0054] A second difference is that there is no integral term with effortreducing ventilatory support, whereas with proportional assistventilation there is such an integral term.

[0055] A third and crucial difference, which follows in part from thesecond difference, is that with effort reducing ventilatory supportthere is no triggering between two states, whereas with proportionalassist ventilation (and most other known forms of ventilatory support)there is such triggering. Specifically, with proportional assistventilation, Equations 2 and 3 define two trigger conditions. Considerfor example the state of affairs at the end of an inspiration using 100%proportional assist, in which R equals the resistance of the subject'sairway, and E equals the elastance of the subject's lungs and chestwall. At this moment, the term Rf(t) is zero (because airflow f(t) iszero), but the term E∫f(t)dt is non-zero, and exactly balances theelastic recoil of the patient's lungs. Since expiration is passive,nothing happens. There is no airflow, and the subject cannot breatheout. It is necessary to switch to Equation 3 in order for the patient tobe able to breathe out. On the other hand, in effort reducingventilatory support (Equation 1), there is no concept of triggeringbetween two states, an inspiratory state and an expiratory state. Againconsider affairs at the end of inspiration. As soon as the subjectstarts to reduce inspiratory effort, the delivered pressure will startto reduce, exactly in parallel with the muscular effort. By the time theinspiratory effort is zero, the mask pressure will have returned to theminimum level P₀, and the degree of support will have returned to zero,as desired, with no need for a trigger. With effort reducing ventilatorysupport the desired pressure-controlling function does not changeabruptly; rather, it changes continuously in proportion to the patient'seffort (i.e., there are no trigger-controlled discontinuities).

[0056] A fourth difference from proportional assist ventilation is thatin proportional assist ventilation it is necessary to either know orempirically determine values for R and E in Equation 2, the subject'sairway resistance, and lung plus chest wall elastance, respectively. Inparticular, the use of values of R or E larger than 100% of thecorresponding physiological values causes unstable run-away of thecontrol algorithm. On the other hand, with effort reducing ventilatorysupport, it is not necessary to know or determine any parameters and, aswill become apparent, even arbitrarily high values of the parameter K inEquation 1 can in principle be used without causing instability orrunaway.

[0057] It is instructive to compare the current invention with PPAP(Proportional Positive Airway Pressure), as taught in Estes U.S. Pat.No. 5,794,615, in which the controlling equation is

P(t)=P ₀ +Kf(t).  (Equation 4)

[0058] Here, the desired variable pressure component is a function ofairflow only. Although there is no trigger in Equation 4, pressure isstill proportional to flow, and not to effort. The difference betweenPPAP and effort reducing ventilatory support is particularly apparent inthe presence of high elastic work of breathing because of the veryabsence of a term in Equation 4 proportional to the integral of flow,which means that with PPAP, only resistive, as opposed to elastic, workis unloaded, whereas with the present invention both resistive andelastic work are unloaded. In addition, PPAP still has the problem ofworking incorrectly in the presence of severe or changing leak, whereaseffort reducing ventilatory support is uninfluenced by leak. As with PAV(Equations 2 and 3), with PPAP (Equation 4) K must be specified to suitthe patient; and large values of K lead to instability, which is not thecase with the present invention.

[0059] In all forms of ventilatory support that include a trigger,factors unrelated to respiratory effort, for example, sensor noise oroscillations in intrapleural pressure due to heartbeat can cause falseor premature switching between the ventilator inspiratory state(typically, a high pressure or inspiratory flow) and the ventilatorexpiratory state (typically, a low pressure or expiratory flow). Witheffort reducing ventilatory support there is no such problem. Instead,such cardiogenic pressure oscillations merely cause minor transientchanges in mask pressure, which approximately cancel out theintrathoracic pressure changes caused by the heartbeat. An interestingsecondary effect is that this will somewhat unload the work of theheart, and this will be of advantage to patients with cardiac failure.

[0060] 4. Degree of Support Adjusted to Servo-Control Effort to be NearZero: Effort Canceling Ventilatory Support

[0061] If the gain K in effort reducing ventilatory support issufficiently high, the control algorithm becomes a simple proportionalservo-controller, in which the patient's respiratory effort is thecontrolled variable, and is servo-controlled to be near zero (effortcanceling—not just reducing—ventilatory support) by increasing the maskpressure if the effort is positive, and decreasing the mask pressure ifthe effort is negative.

[0062] In practice, a simple proportional controller of modest gain (forexample, 10 cmH₂O generated pressure per 1 cmH₂O change in intrapleuralpressure) is adequate, but a PID controller, fuzzy controller, adaptivecontroller, fuzzy adaptive controller, or any other known controllercould also be used, in order to produce somewhat better control. In eachcase, the controller is simply fed with the effort signal E(t) as thecontrolled, or input, variable, the output from the controller is addedto P₀ to achieve the desired instantaneous output pressure P(t), and asuitable pressure request signal is sent to the blower to generate aninstantaneous mask pressure of P(t).

[0063] As previously stated, an advantage of this method is that theeffort signal E(t) does not have to be linear or even calibrated, andcan saturate at high effort, without interfering with useful operation.The only requirements for the device to perform usefully are that theeffort signal should be approximately zero for zero effort, greater thanzero for all positive efforts, and less than zero for all negativeefforts.

[0064] If the effort sensor output is substantially non-zero at zeroeffort, the mask pressure will depart from the desired resting pressureP₀. The circuit of FIG. 3 solves this problem by passing the effortsignal through a trough detector with a time constant long compared witha single breath but short compared with any drift in the zero value forthe effort sensor, and correcting the effort sensor for zero drift bysubtracting the trough signal.

[0065] The other advantages of effort reducing ventilatory support arealso maintained, including no need to customize parameters for aparticular patient, immunity to false triggering from cardiogenicpressure oscillations, and some degree of unloading of the work of thebeating heart.

[0066] 5. Other Effort Sensors and Other Sources of Ventilatory Support

[0067] In the embodiments described above, the effort sensor is anoptical sensor detecting movement of the skin of the suprasternal notch.However, any other form of effort sensor could also be used, forexample, invasively measured pleural or transdiaphragmatic pressure,electromyogram signals from diaphragm, intercostal, or accessoryrespiratory muscles, or electroneurogram signals to these muscles.

[0068] Similarly, the pressure request signal could be used to controlany other kind of ventilatory support device, such as a pneumobelt,rocking bed, cuirasse, iron lung, venturi, or transtracheal ventilator.

[0069] In general, and in particular in all of the above embodiments ofthe invention, the pressure at end expiration, P0, can be setsufficiently high to treat coexisting obstructive sleep apnea. Such apressure can be determined in advance using any conventional manual orautomatic CPAP titration technique. Alternatively, a suitable pressurecan be determined empirically during actual therapy with the currentinvention. During effort-canceling ventilatory support, as described inthe present application, any additional pressure drop across a partiallynarrowed upper airway will be automatically compensated for by an equalincrease in mask pressure, so it is only necessary to set P0 high enoughto prevent passive collapse. The value P0 can be automatically adjustedto treat coexisting obstructive sleep apnea by calculating a measure ofthe conductance of the airway, for example, by using a forcedoscillation method, and increasing P0 if conductance is below athreshold. During ventilatory support with the present invention,obstructive sleep apnea can be distinguished from central sleep apneawith closed vocal cords by inspecting the effort signal. If, during aperiod of zero respiratory airflow (apnea) the effort signal showsongoing inspiratory efforts, then the apnea is obstructive and the endexpiratory pressure should be increased. Conversely, if the effortsignal reveals the absence of effort, then the apnea is central, and ingeneral pressure should not be increased. The determination of thepresence or absence of respiratory effort during an apnea, and thesubsequent increase or non-increase in end expiratory pressure can beperformed automatically. Finally, the value P0 can be automaticallyincreased in the event that the ratio of the effort signal torespiratory airflow is larger than a threshold, indicating anobstructive hypopnea.

[0070] Although the invention has been described with reference toparticular embodiments, it is to be understood that these embodiment aremerely illustrative of the application of the principles of theinvention. Numerous modifications may be made therein and otherarrangements may be devised without departing from the spirit and scopeof the invention.

What is claimed is:
 1. A method for controlling a mechanical ventilatorcomprising the steps of: providing a subject with breathable gas atcontrollable pressure; deriving a measure of respiratory effort from themovement of the skin of the suprasternal notch; and adjusting saidcontrollable pressure in accordance with said measure of respiratoryeffort.
 2. A method for controlling a mechanical ventilator inaccordance with claim 1 wherein the controllable pressure is set to ahigher pressure if the respiratory effort is inspiratory, and set to alower pressure otherwise.
 3. A method for controlling a mechanicalventilator in accordance with claim 1 wherein the controllable pressureis set to a higher pressure if the respiratory effort is inspiratory orif there has not been a transition from low to high pressure for apredetermined period of time since a previous pressure transition, andset to a lower pressure otherwise.
 4. A method for controlling amechanical ventilator in accordance with claim 1 wherein thecontrollable pressure is set to be an increasing function of therespiratory effort, inspiratory effort being taken as positive andexpiratory effort being taken as negative.
 5. A method for controlling amechanical ventilator in accordance with claim 1 wherein thecontrollable pressure is the sum of a first pressure plus a secondpressure proportional to the respiratory effort.
 6. A method forcontrolling a mechanical ventilator in accordance with claim 1 whereinthe controllable pressure is set by a servo-controller, operable toservo-control the respiratory effort to be zero.
 7. A method forcontrolling a mechanical ventilator in accordance with claim 6 whereinthe controllable pressure is made equal to a chosen positive pressurewhen the subject is making no respiratory effort.
 8. A method forcontrolling a mechanical ventilator in accordance with any of claims 1-7wherein said measure of respiratory effort is derived by positioning asensor that includes a light source and a photocell such that thephotocell output is a function of the depth of the subject'ssuprasternal notch.
 9. A method for controlling a mechanical ventilatorcomprising the steps of: providing a subject with breathable gas atcontrollable pressure; deriving a measure of respiratory effort frommovement of the subject's skin; and adjusting said controllable pressureto be the sum of a first pressure plus a second pressure proportional tothe respiratory effort.
 10. A method for controlling a mechanicalventilator in accordance with claim 9 wherein said measure ofrespiratory effort is derived by positioning a sensor that includes alight source and a photocell such that the photocell output is afunction of the depth of the subject's suprasternal notch.
 11. A methodfor controlling a mechanical ventilator comprising the steps of:providing a subject with breathable gas at controllable pressure;deriving a measure of respiratory effort from movement of the subject'sskin; and adjusting said controllable pressure by servo-controlling therespiratory effort to be zero.
 12. A method for controlling a mechanicalventilator in accordance with claim 11 wherein said measure ofrespiratory effort is derived by positioning a sensor that includes alight source and a photocell such that the photocell output is afunction of the depth of the subject's suprasternal notch.
 13. A methodfor controlling a mechanical ventilator so as to reduce the work of theheart, comprising the steps of: providing a subject with breathable gasat controllable pressure; deriving a measure of intrathoracic pressurefrom movement of the skin of the subject's suprasternal notch; andadjusting said controllable pressure in accordance with said skinmovement.
 14. A method for controlling a mechanical ventilator inaccordance with claim 13 wherein said measure of intrathoracic pressureis derived by positioning a sensor that includes a light source and aphotocell such that the photocell output is a function of the depth ofthe subject's suprasternal notch.
 15. A method for controlling amechanical ventilator comprising the steps of: providing a subject withbreathable gas at controllable pressure; deriving a signal that is amonotonic function of the subject's respiratory effort and that exhibitsno discontinuities; and adjusting said controllable pressure inaccordance with said derived signal.
 16. A method for controlling amechanical ventilator in accordance with claim 15 wherein thecontrollable pressure is adjusted to be equal to the sum of a firstpressure plus a second pressure proportional to the derived signal. 17.A method for controlling a mechanical ventilator in accordance withclaim 15 wherein the controllable pressure is adjusted by aservo-controller that operates to servo-control the derived signal to bezero.
 18. A method for controlling a mechanical ventilator in accordancewith any of claims 15-17 wherein the derived signal is approximatelyzero for zero inspiratory effort.
 19. A method for controlling amechanical ventilator in accordance with any of claims 15-17 wherein thederived signal is other than a function of measurement on the airbreathed by the subject.
 20. A method for controlling a mechanicalventilator in accordance with any of claims 15-17 wherein the derivedsignal is approximately zero for zero inspiratory effort, and thederived signal is other than a function of measurement on the airbreathed by the subject.
 21. A mechanical ventilator comprising: asource of breathable gas at a controllable pressure; a transducer formeasuring respiratory effort from the movement of the skin of thesuprasternal notch; and a controller that responds to said transducerand adjusts said controllable pressure in accordance with the measure ofrespiratory effort.
 22. A mechanical ventilator in accordance with claim21 wherein the controller adjusts the pressure to a high level if therespiratory effort is inspiratory, and adjusts the pressure to a lowlevel otherwise.
 23. A mechanical ventilator in accordance with claim 21wherein the controller adjusts the pressure higher if the respiratoryeffort is inspiratory or if there has not been a transition from low tohigh pressure for a predetermined period of time since a previouspressure transition, and adjusts the pressure lower otherwise.
 24. Amechanical ventilator in accordance with claim 21 wherein the controlleradjusts the pressure to be an increasing function of the respiratoryeffort, inspiratory effort being taken as positive and expiratory effortbeing taken as negative.
 25. A mechanical ventilator in accordance withclaim 21 wherein the controller adjusts the pressure to equal the sum ofa first pressure plus a second pressure proportional to the respiratoryeffort.
 26. A mechanical ventilator in accordance with claim 21 whereinthe controller adjusts the pressure by operating a servo-controller thatservo-controls the respiratory effort to be zero.
 27. A mechanicalventilator in accordance with claim 26 wherein the controller adjuststhe pressure to equal a chosen positive pressure when the subject ismaking no respiratory effort.
 28. A mechanical ventilator in accordancewith any of claims 21-27 wherein the measure of respiratory effort isderived by positioning a sensor that includes a light source and aphotocell such that the photocell output is a function of the depth ofthe subject's suprasternal notch.
 29. A mechanical ventilatorcomprising: a source of breathable gas at a controllable pressure; atransducer for measuring respiratory effort from movement of a subject'sskin; and a controller that responds to said transducer and adjusts saidcontrollable pressure to be the sum of a first pressure plus a secondpressure proportional to the respiratory effort.
 30. A mechanicalventilator in accordance with claim 29 wherein said measure ofrespiratory effort is derived by positioning a sensor that includes alight source and a photocell such that the photocell output is afunction of the depth of the subject's suprasternal notch.
 31. Amechanical ventilator comprising: a source of breathable gas at acontrollable pressure; a transducer for measuring respiratory effortfrom movement of a subject's skin; and a controller that responds tosaid transducer and adjusts said controllable pressure byservo-controlling the respiratory effort to be zero.
 32. A mechanicalventilator in accordance with claim 31 wherein said measure ofrespiratory effort is derived by positioning a sensor that includes alight source and a photocell such that the photocell output is afunction of the depth of the subject's suprasternal notch.
 33. Amechanical ventilator that reduces the work of the heart comprising: asource of breathable gas at a controllable pressure; a transducer forderiving a measure of intrathoracic pressure from movement of the skinof the subject's suprasternal notch; and a controller that responds tosaid derived measure and adjusts said controllable pressure inaccordance with said skin movement.
 34. A mechanical ventilator inaccordance with claim 33 wherein said measure of intrathoracic pressureis derived by positioning a sensor that includes a light source and aphotocell such that the photocell output is a function of the depth ofthe subject's suprasternal notch.
 35. A mechanical ventilatorcomprising: a source of breathable gas at a controllable pressure; atransducer for deriving a signal that is a monotonic function of asubject's respiratory effort and that exhibits no discontinuities; and acontroller that adjusts said controllable pressure in accordance withsaid derived signal.
 36. A mechanical ventilator in accordance withclaim 35 wherein said controller adjusts the pressure to equal the sumof a first pressure plus a second pressure proportional to the derivedsignal.
 37. A mechanical ventilator in accordance with claim 35 whereinsaid controller adjusts the pressure by servo-controlling the derivedsignal to be zero.
 38. A mechanical ventilator in accordance with any ofclaims 35-37 wherein the derived signal is approximately zero for zeroinspiratory effort.
 39. A mechanical ventilator in accordance with anyof claims 35-37 wherein the derived signal is other than a function ofmeasurement on the air breathed by the subject.
 40. A mechanicalventilator in accordance with any of claims 35-37 wherein the derivedsignal is approximately zero for zero inspiratory effort, and thederived signal is other than a function of measurement on the airbreathed by the subject.